Such fill level measuring devices working according to the travel time principle are widely applied in industrial measurements technology.
A measuring device group of this type applied in industrial measurements technology is formed by fill level measuring devices working with microwaves according to the pulse radar method. Such devices are sold, for example, by the assignee under the mark Micropilot.
In the pulse radar method, short microwave pulses with frequencies in the gigs hertz range are periodically sent toward the substance with a predetermined repetition frequency, e.g. a repetition frequency having an order of magnitude of 1 to 2 MHz, and their signal components reflected back in the container in the direction of the transmitting and receiving system are received after a travel time dependent on the traveled path. In such case, regularly based on the received signal, an auxiliary signal is derived, which shows amplitude and phase information in the received signal as a function of associated travel time.
Due to the high signal frequencies and the, as a rule, very short travel times, respectively travel time differences, to be resolved, for this, an auxiliary signal, designated frequently as an intermediate frequency signal, is generated, which is a version of the received signal expanded in time. A corresponding method is described, for example, in European Patent EP 1 324 067 A2. As likewise described there, the auxiliary signal is usually subsequently rectified and fed via a low-pass filter and an analog-digital converter to an evaluation unit. Since the amplitude of the received signals decreases with the square of the traveled path, the received signal can have strongly different amplitudes. In order better to handle this situation, the auxiliary signal is preferably supplementally transformed into a logarithmic representation. The measuring of the travel time of the signal fraction reflected on the surface of the substance occurs by determining an envelope of the rectified, log, filtered and digitized, auxiliary signal. This envelope is frequently referred to as the amplitude envelope curve. The envelope curve is a plot of amplitude of the time expanded, received signal as a function of the travel time. For any given reflector, the envelope curve will show a maximum at the travel time required for the signal to travel the path from the transmitting and receiving system to the reflector and back. Correspondingly, the sought separation results directly from the travel time of the maximum of the envelope curve and the propagation velocity of the used signals.
It is known to improve the accuracy of measurement of such fill level measuring devices by conducting, besides the described evaluation of the amplitudes of the received signal, respectively the intermediate signal derived therefrom, supplementally a determination of a phase difference between the transmitted and received signals, and to use such for correcting the amplitude determined travel time of the signal fraction reflected on the surface of the substance.
Such methods and measuring devices are described, for example, in German Patent, DE 44 07 369 A1 and Published International Application, WO 02/065066 A1. Determining the phase difference between the transmitted and received signals requires, as a rule, relatively complex circuits and evaluation methods.
A clearly more cost, and energy, efficient method as regards circuitry and evaluation is known from European Patent, EP 1324 067 A2. Described there is a method for measuring fill level of a substance located in a container, wherein, in measuring cycles following one after the other,                signal pulses of predetermined frequency are sent by means of a transmitting and receiving system with a predetermined repetition frequency into the container, and their signal components reflected back in the container in the direction of the transmitting and receiving system are received as received signal after a travel time dependent on their traveled path,        based on the received signal, an auxiliary signal is derived reflecting amplitude and phase information of the received signal as a function of travel time,        based on the auxiliary signal, a travel time of a signal fraction reflected on the surface of the substance is determined as fundamental travel time,        travel times of zero crossings of the auxiliary signal are determined,        based on the travel times of the zero crossings, a phase difference between the transmitted signal and the received signal is determined,        based on the phase difference, a correction of the measured fundamental travel time is performed, and        fill level is determined based on a propagation velocity of the signal pulse, an installed height of the transmitting and receiving system above the container and the corrected fundamental travel time.        
In such case, the fundamental travel time is determined also here as travel time of a maximum of an envelope curve of the rectified, log, filtered and digitized auxiliary signal attributable to the reflection on the surface of the substance. In parallel therewith, the phase difference between the transmitted signal and the received signal is derived based on the log auxiliary signal. For this, the logarithmic auxiliary signal is differentiated via a differentiating stage twice with respect to travel time. Provided on the output of the differentiating stage is therewith an output signal, which has marked peaks at the travel times corresponding to the zero crossings of the auxiliary signal. Therewith, the travel times of the zero crossings and thus the phase shift of the received signal can be determined, without having to digitize the output signal. The peak-amplitudes can be normalized, for example, with the assistance of a Schmitt-trigger and the associated travel times registered with the assistance of a timer. This provides a circuit-wise very simply and cost effectively implementable method consuming little energy for determining the phase difference between transmitted and received signals.
In the case of registering the phase difference between transmitted and received signals for measurements, there results the problem that the phase shift of the received signal, respectively of the auxiliary signal, relative to the associated transmitted signal can also be slightly different from measuring cycle to measuring cycle even in the case of unchanged fill level. Cause for this scattering of the measured phase differences are time shifts caused in the circuit and/or in the signal processing both in the direct time relationship between transmitted signal and received signal as well as also between the received signal and therefrom derived additional signals, based on which the phase difference is lastly determined. In such case, the absolute value of a systematically arising constant time shift is, as a rule, uncritical, since it can be determined by reference measurements and correspondingly compensated. In contrast, random fluctuations of the time shift cannot be registered and accordingly also cannot be compensated. Such lead, thus, to a limitation of the achievable accuracy of measurement with which the phase difference and therewith naturally also the fill level can be determined.
Another measuring device group of this type applied in industrial measurements technology is formed by ultrasonic fill level measuring devices working according to the pulse travel time method. The latter are sold, for example, by Endress+Hauser under the mark, PROSONIC. Also in this case, short ultrasonic pulses of predetermined frequency and duration are sent with a predetermined repetition frequency by means of an ultrasonic transducer and a corresponding transmitting and receiving system and their signal components reflected back to the transmitting and receiving system received after a travel time dependent on the traveled path. The frequencies of the ultrasonic pulses lie here, as a rule, in the region of 1 kHz until 200 kHz, so that a time expansion of the received signal received via the ultrasonic transducer is not required. Typically here, an auxiliary signal is derived, which corresponds to the amplified received signal. Apart from this difference concerning the lower signal frequency, the other signal processing occurs, in principle, the same as for fill-level measuring devices working with microwaves. I. e., also here, the auxiliary signal is digitized by means of an analog-digital converter, in given cases, transformed into a logarithmic representation, and an envelope curve derived, which shows the amplitudes of the received signal as a function of the associated travel time required for the path from the transmitting and receiving unit to the respective reflector and back. Based on the envelope curve, also here, the maximum of the envelope curve to be attributed to the reflection on the surface of the substance is determined, and, based on its travel time, the fill level is calculated.